The present invention relates to a process for producing composite panels of the sandwich type, as well as to the sandwich composite panels obtained by implementing said process.
It is known that composite panels of the sandwich type consist of a thick cellular core (for example of the honeycomb type) fastened to two external skins, formed by fibers impregnated with polymerized resin and arranged on either side of said cellular core. In addition, it is known that such panels are being more and more frequently used, especially because of their high rigidity and their great dimensional stability, for producing structural elements, for example those intended for aircraft.
The cellular core of these panels may be metallic or organic, with honeycombed cells of square or hexagonal cross section. Such a cellular core has a high compressive strength in a direction orthogonal to its plane, but is not very good at withstanding compression parallel to its plane. As regards said skins, these may each be formed from plies composed of several superposed layers and consisting of fibers impregnated with thermosetting resin. The fibers constituting these plies may be woven or nonwoven and they are oriented in directions chosen according to the desired mechanical properties of the panels. Such fibers may be carbon fibers, glass fibers, kevlar (registered trademark) fibers, etc., depending on the application envisaged.
In order to manufacture such sandwich composite panels, the cellular core and the plies which cover its faces are placed inside an autoclave or similar device, in which the panel preform, consisting of said core and said plies, is subjected to a hot polymerization cycle under pressure. The pressure is, for example, obtained by placing said panel preform in an airtight case or vacuum bag in which a partial vacuum (a few tenths of a bar) is created and by applying a certain pressure to the outside of this airtight case (a few bar).
Along the borders of the panel, the plies covering both faces of the core and the flank of said cellular core are extended beyond the peripheral edge of said core and they are fastened together during polymerization, so as to create a peripheral border. Such a peripheral border enables the panel to be fixed to a support frame or to another panel by any fixing means such as bolts, rivets or adhesive.
When the panel preform is pressurized during polymerization, the periphery of said cellular core is critical region as regards correct adhesion of the skins to this core. The reason for this is that, in this region, the force due to the pressure exerted on the preform comprises a lateral component, oriented inwards parallel to the faces of the panel, which the cellular core cannot oppose with a sufficient counterpressure on account of its low compressive strength in this direction. Consequently, the cellular core tends to deform locally at its periphery, so that a satisfactory bond between the skins and this core cannot be obtained, such deformation increasing as the dimensions of the cells of said core increase.
Attempts have already been made to remedy this drawback.
For example, it has already been considered to deposit a film of intumescent adhesive on the edge of the cellular core. Thus, during hot polymerization, the intumescent adhesive increases in volume and fills the cells. In order to prevent any deformation occurring before this product has become sufficiently rigid, a system of strips is arranged laterally over the entire periphery of the panel and serves to transmit the pressure to the edges of the panel during polymerization. Such a known method of stiffening the borders results in an appreciable increase in the weight of the composite panels since the weight of the intumescent product approximately 80 grams per linear meter. In addition, the necessity of using a system of strips leads to having to use complicated tooling and to a relatively long manufacturing time, and consequently to an excessive cost which is further accentuated by the difficulties in automating this process in order to make it suitable for industrial-scale production.
As regards Patent EP-A-0,311,931, this describes a process consisting in arranging a honeycombed frame, the cells of which are arranged in a direction inclined with respect to the plane of the panel, on the periphery of the honeycombed core and in cutting this frame in a bevel so as to form a peripheral flank substantially normal to this inclined direction. Such a process enables a panel to be obtained in which the flank is reinforced without any increase in weight, but the manufacturing time remains relatively long, the cost of the panel remains relatively high and automated manufacture is difficult to implement.
Moreover, Patent FR-A-2,658,116 describes a process consisting in cutting the edge of said cellular core in a bevel, in depositing a film of hotmelt adhesive on the edge thus cut, in heating this film of adhesive so as to liquefy the latter and to cause it to be deposited by capillary effect on those walls of said core which are adjacent to the cut edge, and then in cooling the hotmelt adhesive so as to cause it to become rigid and to stiffen the walls of the core adjacent to the cut edge.
This latter process enables the edge of said core to be sufficiently strengthened so that the latter can support the pressure applied during polymerization, without incurring too high a weight penalty, and for a manufacturing time which is substantially shorter than that for the other known processes, so as to result in a significant reduction in the manufacturing cost and in the scrap rates, it being possible furthermore for this process to be automated within the context of industrial-scale manufacture of the panels. However, such a process requires the edge of the core to be cut in a bevel, the angle of said bevel preferably being between 25.degree. and 40.degree..